1 The horizontal rate of flow of mass through a plane normal to the direction of the horizontal velocity vector. Depending on the context, the flux may be through an element of area at a given position in the vertical plane, through a unit of width extending from the glacier bed to the surface, or through an entire glacier cross section. 2 The vertical rate of flow of mass at the glacier surface or bed. In sense 2, the flux at the surface is equal to the sum of surface accumulation and surface ablation, or in other words to the surface mass balance. Equivalently the flux at the bed is equal to the basal mass balance.